DESCRIPTION: Early language delay is a costly public health problem because of its clinical, educational, and societal implications. For example, children who have small vocabularies have a greater likelihood of developing language impairments (a disorder that affects 6-7% of children in the US), are more likely to lag in their readiness for school, and have a greater chance of developing behavioral problems. Research has revealed that children's early language experiences are a major predictor of their language development. How, and through what mechanisms, these experiences lead to robust language development though is still not well understood. One barrier to progress in this area of research is that children's language experience is a multi- dimensional construct. It includes: the quantity of the words children hear, the nature of the interactive contexts in which those words occur, and the referential transparency evident in child-directed speech. All of these dimensions of language experience have been individually linked to vocabulary development. Little work, however, has sought to understand how different dimensions of language experience together jointly shape language development, specifically early vocabulary development. The proposed research plan is aimed at developing a unified understanding of the role of language experiences on early language development. Three specific aims are proposed. The first aim is to understand how the dimensions interconnect with one another and how they uniquely and jointly contribute to language development. The second aim is to understand whether the roles the dimensions play in language acquisition change with development. The final aim is to uncover whether different dimensions of experience impact different children in different ways. The knowledge gained from pursuing these three aims will lead to a deeper understanding of one of the most important sources of variability in language development. Additionally, because modifications to the language experience is a common method of intervention for children with language delay, the results from this research may inform ways of developing more effective intervention strategies. In pursuing these research objectives, the applicant will complete a tailored set of scientific and professional development activities that go beyond the applicant's current training. This will include training in the translational science of language acquisition, training in the developmental science of early social processes, and extensive training in developmental methodologies and statistics. This skill set, coupled with the applicant's previous training, will prepare the applicant to launch an independent research career that is advanced theoretically, cutting edge methodologically and that bridges the gap between the basic and applied sciences of early language development.